The motif for peptide binding to the insulin-dependent diabetes mellitus-associated class II MHC molecule I-A(g7) validated by phage display library

The MHC class II molecule I-A(g7) is essential for the development of insul in-dependent diabetes mellitus (IDDM) in the non-obese diabetic (NOD) mouse but the requirements for peptide binding to I-A(g7) are still controversia l, We have now isolated I-A(g7)-binding phage from a large phage display li brary encoding random nonamer peptides, Ninety peptide-encoding regions of phage eluted from I-A(g7) were sequenced and >75% of the corresponding synt hetic peptides bound to I-A(g7). Peptide alignment led to the identificatio n of position-specific anchor residues. Hydrophobic (V and P) and positivel y charged (K) residues were highly enriched at P6 and positively charged (R and K), aromatic (Y) or hydrophobic (L) residues at P9, In addition, small amino acid residues (G and A) were enriched at P7 and G at P8. The primary anchors at P6 and P9 defining the phage-derived motif were present in most high-affinity I-A(g7)-binding peptides from IDDM candidate antigens but on ly in less than or equal to 25% of peptides that were low-affinity binders or failed to bind to I-A(g7). A comparison of these results with the propos ed motifs for peptide binding to I-A(g7) validates the one we have previous ly described.